The memory areas (I/O2 + I/O3) are naturally for 2K of I/O registers, but they was used also for other applications, and it's very interesting because isn't common find some software there, SW will be "hidden" and probably compatible with the presence of other SW....

Thus far no-one had bothered to do something about this. Most probably because with SJLOAD there is now a working option available for JiffyDOS enabled drives. And PAL users could also take HYPRA-SYSTEM, which not only accelerates LOAD but also SAVE, and works on non-modified 1541/1571 drives - but sadly neither on 1581 nor (at least for the time being) SD2IEC drives.

HYPRA-SYSTEM is included with Mega-Cart, but can also be soft-loaded. It requires more than 2K, and so wouldn't be a good candidate for putting it into I/O 2/3 anyhow. It could however be assembled to sit at $0400..$0FFF, which would also be a good choice for most 8K+ programs.

And here's my opinion about the use of I/O 2/3 for anything other than as cartridge I/O resource.

Before 2006, I made a modded version of Compute's Gazette turbodisk which was designed to reside at the range used by IO2/3. However, it was only "relocated" there and required RAM to be available in there so I used NVRAM in these 2k.

Concerning the Rabbit tape accelerator, I have the original cart and it wasn't designed at all to operate in the IO2/3 region originally. The real Rabbit used $7000-$7800.

I am quite sure what you find on Zimmers (formerly FUNET) is a modded version but I must say its a mod that makes perfect sense. The $7000-$7800 area is used by a lot of other utility software.

The Behr-Bonz multicart for VIC-20 has a control register located in the IO3 area.

brain wrote:I don't know of any apps that live there, but UltiMem supports RAM there, (except for 16 bytes at top of IO), which might make it more attractive to put stuff there.

If an application is going to use the 2K at $9800..$9FFF, it probably needs it all, so excluding the top 16 bytes would only be doing half the job. There should be a provision to switch off the control registers entirely (like the MMU of the C128 does in its C64 mode), so the cartridge looks like a "normal" RAM expansion until the next power cycle or external reset.